1. Field of the Invention
The present invention relates to focal-plane shutters and, more specifically, to a double-shielding type focal-plane shutter for performing an exposure operation.
2. Background Information
In a focal-plane shutter disclosed in Japanese Utility Model Laid-Open No. 29839/1991, for example, an exposure operation is conducted by alternately reciprocating an opening shutter blade unit and a closing shutter blade unit over a shutter opening provided on a base plate. The opening and closing blade units are driven by a drive mechanism provided on a rear side of the base plate.
Shutters are classified into a single-shielding type shutter and a double-shielding type shutter in accordance with the manner of movement of the opening and closing blade units. In an initial state of a single-shielding type shutter, a closing blade is in a state in which it retreats from a shutter opening, and an opening blade is in a closed state. In contrast, in an initial state of a double-shielding type shutter, both the closing blade and the opening blade close a shutter opening.
In some single-shielding type focal-plane shutters, a closing blade, which should inherently retreat from a shutter opening in an initial state, acts to close the shutter opening due to shock or some other reason before an opening blade opens the shutter opening. In this state, when the closing blade closes the shutter opening, a closing arm for transmitting a drive force to the closing blade abuts against a pin of an opening lever which drives the opening blade, as shown in FIG. 5, to forcedly drive the opening blade from a holding state to retreat the same from the shutter opening. By the foregoing construction, before a control circuit performs shutter driving control, a flash timing switch is switched ON by the opening lever so that the control circuit can detect an abnormal operation of the shutter.
In double-shielding type focal-plane shutters, there is also the possibility that when a closing blade retreats from a shutter opening after a release operation, the closing blade closes the shutter opening due to some reason before an opening blade retreats from the shutter opening as in the single-shielding type shutter.
When the arrangement described above for the single-shielding type shutter, in which a closing arm is caused to abut against a pin of an opening lever to forcedly drive an opening blade, is applied to the double-shielding type shutter, there results a situation in which at the time of charging the shutter to an initial position, the pin of the opening lever abuts against the closing arm, thereby making it impossible for the opening blade to be returned to the initial position. The reason for this is that while in the single-shielding type shutter the opening lever cannot abut against the closing arm since the closing blade retreats from the shutter opening in an initial position, in the double-shielding type shutter both the opening blade and the closing blade close the shutter opening in an initial position. Thus, the foregoing arrangement employed in single-shielding type shutters cannot be incorporated in double-shielding type shutters. As a result, certain abnormal operations of double-shielding type shutters cannot be effectively detected.
It is an object of the present invention to provide a double-shielding type focal-plane shutter which can effectively detect an abnormal operation of the shutter by providing a closing lever having a drive pin at one end thereof for driving a closing blade and an engagement portion at another end thereof for engaging first and second mating engagement portions of a lock lever for restricting movement of the closing lever.
It is another object of the present invention to provide a double-shielding type focal-plane shutter in which engagement between the second mating engagement portion of the lock lever and the engagement portion of the closing lever places the closing blade in a closed position, in which such engagement is released immediately before the shutter returns to an initial condition, and in which engagement between the engagement portion of the closing lever and the first mating engagement portion of the lock lever causes the closing blade to retreat slightly from the closed position to prevent the drive pin of the opening lever from abutting against a closing arm of the closing blade.
The foregoing and other objects of the present invention are carried out by a double-shielding type focal-plane shutter comprising a closing blade and an opening blade for closing and opening a shutter opening, a closing lever for driving the closing blade, an opening lever for driving the opening blade, a lock lever for restricting motion of the closing lever, and a set lever for setting the closing lever, the opening lever and the lock lever in predetermined positions. The closing lever comprises a first closing lever and a second closing lever. The second closing lever and the first closing lever are supported coaxial with each other and are mounted for undergoing pivotal movement. The second closing lever is provided at one end thereof with a drive portion for driving the closing blade and at the other end thereof with an engagement portion for engaging with the lock lever. The lock lever has a first mating engagement portion and a second mating engagement portion. The engagement portion of the second closing lever engages with the second mating engagement portion of the lock lever short of a position where it is set in the predetermined position and engages with the first mating engagement portion of the lock lever to be set in the predetermined position.
Preferably, the engagement portion of the second closing lever is positioned to engage the second mating engagement portion of the lock lever after termination of an exposure operation and to thereafter release the second mating engagement portion when the set lever sets the closing lever, the opening lever and the lock lever to thereby engage the first mating engagement portion of the lock lever to hold the second closing lever in the predetermined position. Engagement between the first mating engagement portion of the lock lever and the engagement portion of the second closing lever is released when the set lever moves to a retreat position from the set position.